Electroluminescence is the emission of light from a crystalline phosphor due to the application of an electric field. A commonly used phosphor material is zinc sulfide which may be activated by the introduction of various elements such as manganese into its lattice structure. When such a material is subjected to the influence of an electric field of a sufficient magnitude, it emits light of a color which is characteristic of the composition of the phosphor.
Two major subdivisions of electroluminescent devices are defined in terms of the intended alternating current (AC) or direct current (DC) operating modes. In DC configurations, the phosphor pixels of the panel are caused to luminesce in response to the conduction of electricity through the pixels. In AC configurations, the pixels luminesce in response to capacitively coupled electrical energy. AC electroluminescent devices may be made in either a thin-film or a thick-film configuration. In the thick-film configuration, powder phosphors are formed by precipitating powder phosphor crystals of the proper grain size, suspending the powder in a lacquer-like vehicle, and then applying the suspension to a substrate, for example, by spraying, screening or doctorblading techniques. Thin-film phosphors are grown from condensation of evaporants from vacuum vapor depositions, sputtering or chemical vapor depositions.
The present invention has particular applicability in relation to thick-film and thin-film AC electroluminescent matrix displays. Such matrix display panels can be used for a variety of applications, and in general, can find utility as substitutes for cathode ray tubes (CRTs), wherever CRTs are used. For example, matrix display panels can be used for such applications as oscilloscopes, television sets and monitors for computers. An electroluminescent matrix display panel is desirable because it provides a flat panel display which is much more compact than a corresponding CRT display.
AC matrix electroluminescent displays having, for example, 640 column.times.200 row matrix-addressable display pixels have been provided in the industry. Such displays are conventionally energized by sequentially scanning the 200 rows of the display and, as each row is scanned, applying column data for the scanned row. This technique is known as multiplexing In such a conventional pulsed energization scheme, each pixel of the panel is turned on or is energized with a duty cycle which is approximately 0.005. Such pulsed matrix displays have been known to provide images having an average intensity of approximately 30 foot lamberts (FL). This relatively low intensity image is produced by having the pixels radiate at an instantaneous amplitude of about 6,000 FL. Although a monochromatic display of manganese doped zinc sulfide is capable of providing such luminance in response to pulsed energization, it has not heretofore been possible to provide such luminance with red, green, blue (RGB) electroluminescent phosphors. Accordingly, although relatively satisfactory pulsed monochrome AC electroluminescent displays have been provided, it has not been possible to provide a color display with a suitable luminance.
Certain display technologies have resolved the problem of low luminance pulsed energization by providing active energization matrices which continuously energize an addressed pixel and therefore operate the pixel with a duty cycle of unity. Such constantly energized pixels can provide a total luminance in a matrix display which is orders of magnitude greater than would be provided in a pulsed energization mode.
Some liquid crystal displays (LCDs) utilize thin-film transistors or diodes and capacitors at each pixel of the display to maintain a charge on each addressed pixel and therefore provide for constant energization and illumination of the pixel. Although color images have been provided with such displays, it has been extremely difficult to provide the high density of thin-film transistors which are required to energize pixels in such a fashion. Indeed, at the present state of the art, although large screen displays are possible in principle, it has been difficult to manufacture such displays reliably and inexpensively. Also, although the thin-film driving transistor approach may be suitable for use with low power LCDs, such an approach is not considered practical for use with electroluminescent displays which require a substantial amount of power to provide a desired luminance. Accordingly, it is not desirable to drive the pixels of an electroluminescent display by using thin-film transistors.
AC gas discharge matrix-addressed displays have been designed to operate so that each addressed pixel may be energized and lighted with a 100 percent (unity) duty cycle. In such a device, each pixel continuously receives a high frequency sustaining voltage of, for example, 10-50 khz which is below the ignition level of plasma gas located in the envelope of the display. An unaddressed pixel therefore remains off. However, when any pixel is addressed by applying momentarily an additional write voltage which is properly synchronized with the sustaining voltage, the gas is ionized and ignition takes place at that pixel. Moreover, due to the sustained pixel wall voltage and the decay time of the ionized gas, that pixel, once addressed, continues to conduct in an AC sense until it is turned off by a synchronized erase voltage pulse of sufficient amplitude and proper polarity. The erase pulse momentarily reduces the wall voltage to a value below that required to sustain ionization.
Thus, once turned on, a pixel of a properly designed AC gas plasma display remains on until turned off. It operates at 100 percent duty cycle in an AC sense, and requires no connections other than its two power leads to operate in this fashion.
Gas discharge/phosphor hybrid displays have been proposed. Such displays have attempted to use an ionized plasma to excite a phosphor to luminescence. Thus, there have been attempts to construct a hybrid display wherein electrons derived from ionization of the plasma are directed to energize phosphor pixels It has also been suggested that ultraviolet light given off by a plasma could be used to energize phosphor pixels. One disadvantage of such hybrid displays is that the UV and ion bombardment degrades the phosphor. The literature does not report success with such plasma energization schemes.
Known displays have not been able to successfully combine the advantageous 100 percent duty cycle gas discharge drive technology with electroluminescent display elements The gas discharge switched electroluminescent (EL) display of the invention combines the advantageous features of gas discharge and EL technology by using gas discharge elements as switches in series with associated EL display cells. Thus, in the display of the invention, AC EL display elements and gas discharge switch elements are capacitively coupled but the phosphor is isolated from the plasma. In operation, the AC gas discharge elements act as on/off switches for the corresponding EL display cells and thereby provide a 100 percent duty cycle when the switches are turned on. None of the known display technologies uses this construction wherein each gas discharge switch controls the energy delivered to its associated isolated EL cell.
Accordingly, it is an object of the invention to provide a display which utilizes memory (i.e., continuous) gas discharge AC drive technology in association with AC electroluminescent display technology.
It is a further object of the invention to provide such a display wherein AC matrix gas discharge switches turn on and remain on, thus providing continuous energization for corresponding series connected and capacitively coupled AC EL cells.